The purpose of this project is to further define the role of cerebrovascular factors and their contribution to secondary ischemia following closed head injury. In order to do this, transcranial Doppler (TCD) ultrasonography will be utilized to study several cerebrovascular phenomenon which can contribute to secondary ischemia. These phenomena are: 1) cerebral vasospasm 2) alteration of cerebral autoregulation 3)changes in cerebrovascular tone leading to alterations in control of the circulation. 1. Vasospasm - TCD ultrasonography will be used to identify patients with vasospasm following severe closed head injury. Single photon emission computerized tomography (SPECT) and CT scanning will be used to assess the effect of vasospasm on reduction of cerebral blood flow and production of delayed infractions. 2. Cerebral autoregulation - TCD has recently been shown to be capable of evaluating the autoregulatory response noninvasively. This represents a major advance in technology for this purpose. It is proposed that this method be further confirmed by comparing it to other methods of measuring cerebral blood flow continuously and then be utilized to address some of the relevant issues concerning autoregulation in head injured patients. It is proposed that patients with impaired autoregulation be studied longitudinally to determine when autoregulation returns back to normal. This will have significant implications in the treatment of head injured patients. 3. Altered cerebrovascular tone - a new method has been developed to examine pressure flow relationships in the cerebral circulation noninvasively using TCD. By further confirming this method, it can be applied to the study of cerebrovascular tone in head injured patients. This may lead to significant insights concerning factors responsible for determining the resistance of the cerebral circulation following head injury. It may also be possible to determine the intracranial pressure non-invasively from these measurements.